- is there also Scandisk & disk defragment in linux?

Linux "scandisk" is fsck (which stands for File System ChecK).

fsck is usually run as necessary when your system boots and your filesystems are about to be mounted read-write.

Both fsck and defragmenting utilities are filesystem-specific. While all (as far as I know) linux filesystems provide a fsck utility, most linux filesystems do not have a defragmenting utility, because they are designed to avoid fragmentation, and any fragmentation is usually too low for the process of defragmentation to be considered worthwhile.

As long as you have a reasonable amount of free space, your filesystem should not really become fragmented.

EDIT: If you're interested, the filefrag utility can be used to inspect the fragmentation (if any) of a specific file. See man filefrag.

-ok that good...
-so how can i use the fsck command to check my hdd?

su to root, run fsck. It's that simple.

Be careful - it's BAD to run fsck on a filesystem that is mounted read-write for obvious reasons (what if you change something while fsck is fixing something?) You'll probably be warned about this when you run fsck.

See man fsck.